When operating microscopes are used in eye surgery, particularly in cataract removal, the angle between the illumination axis and the observation axis of the microscope should be kept as small as possible. The advantage of this type of illumination is that the rays of light falling perpendicularly onto the eye are reflected diffusely by the retina and as a result the lens capsule of the eye appears as a translucent reddish colour. This effect is also known as the red reflex. The quality of this red reflex is of critical importance in cataract removal. In this operation, after the lens has been removed, all tissue residues also have to be removed from the eye. This can only be done successfully if these tissue residues are shown up with sufficient optical contrast, and this is the purpose for which the red reflex is used.
However, in the course of a typical operation, not only is it essential to show the red reflex, but there must also be conventional illumination (as required) to illuminate the operating area.
From EP 0 661 020 B1, an illuminating device for an operating microscope is known, having at least one light source, a collector lens system of a luminous field diaphragm or aperture, optical deflectors, other lenses and a main objective, the illuminating light being guided through the main objective into the object plane. The luminous field diaphragm is imaged through the deflectors and the lenses are imaged through the main objective into the object plane. Moreover, to produce red reflex illumination, switchable means are provided in the illumination beam path which store the image of the luminous field diaphragm and image the light source into the object plane.
From DE 4 326 761 a stereomicroscope for ophthalmic surgery is known, for high-contrast visualisation of transparent media in the reflected transmitted light, wherein phase contrast elements are arranged in the two observation beam paths which image transparent media as phase objects with amplitude contrast. The provision and positioning of phase contrasting elements of this kind is regarded as laborious.
One particularly disadvantageous aspect of phase contrast observation of this kind is that the phase contrasting elements typically configured as diaphragms (apertures) have to be introduced into the observation beam of the microscope. Expediently, positions are provided for this purpose at which the first orders of refraction of the secondary scattered light source produce the image. It is difficult to achieve sufficiently accurate positioning with the complex illumination and imaging processes, including the patient's eyes that are to be observed, which may vary greatly both in terms of dimensions and nature.
The provision of different diaphragm shapes and sizes using LCD (liquid crystal device) with individually controllable pixels is known from DE 196 44 662 A1, although this publication does not describe any applications in ophthalmic surgery.